Interlayer charge transport controlled by exciton–trion coherent coupling

Abstract

The possibility of electrical manipulation and detection of charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. Using a few-layer graphene/monolayer WS 2 /monolayer graphene vertical heterojunction, we report inter-layer charge transport from top fewlayer graphene to bottom monolayer graphene, mediated by coherently formed trion state. This is achieved by using a resonant excitation and varying the sample temperature, the resulting change in the WS 2 bandgap allows us to scan the excitation around the exciton-trion spectral overlap with high spectral resolution. By correlating the vertical photocurrent and in situ photoluminescence features at the heterojunction as a function of the spectral position of the excitation, we show that (1) trions are anomalously stable at the junction even up to 463 K due to enhanced doping, and (2) the photocurrent results from the ultra-fast formation of trion through exciton-trion coherent coupling, followed by its fast inter-layer transport. The demonstration of coherent formation, high stabilization, vertical transportation and electrical detection of trions marks a step towards room temperature trionics. 1 arXiv:1903.06437v1 [cond-mat.mes-hall]

Gate-tunable trion switch for excitonic device applications

“…One could readily identify that apart from a red shift of the peak position due to temperature induced reduction in bandgap, the trion peak survives up to the highest temperature (423 K) used in the experiment -suggesting highly stable trion on the junction. This is further supported by an enhancement in the trion dissociation energy (separation between the X 0 and X − peaks) at higher temperature in Figure 1e, arising from enhanced doping of monolayer WS 2 at higher temperature [25].…”

“…An external gate control has also been achieved by modulating the binding energy of the neutral exciton [4,5].In this regard, the charged exciton or trion (X − ) is promising since its intensity can be readily controlled electrically by modulating the doping density using a gate voltage. In addition, the trion, while being optically initiated, can be electrically detected in a spatially nonlocal manner through measuring a charge current [25]. The relatively longer radiative lifetime of trion compared to intra-layer exciton [6,11] further helps in nonlocal detection.…”

Anomalous Stark Shift of Excitonic Complexes in Monolayer Semiconductor

“…δε n is primarily dependent on the doping induced Pauli blocking. 45 In the current structure, where WS 2 remains floating, we do not electrostatically dope the system by the external voltage and thus δε n plays the role of an additive constant. The change in ∆ε thus provides a direct measure of the reduction in trion binding energy at different V ext .…”

Highly Tunable Layered Exciton in Bilayer WS2: Linear Quantum Confined Stark Effect versus Electrostatic Doping

“…Similar observations are reported in monolayer as well, 57,58 which is usually attributed to the enhancement in the energy required to move the additional electron (for X − ) or hole (for X + ) to respective bands due to doping induced Pauli blocking. 52,59 The extracted oscillator strength of X 0 and X ± in Figure 5e shows a doping dependent transfer of oscillator strength from one excitonic species to another. At low V g , when the bilayer Fermi level is deep inside the band gap, X 0 formation is favoured.…”

“…The simple Hamiltonian described in Supporting The intra-layer A (2) 1s exciton in bilayer WS 2 has fast non-radiative scattering channels to lower lying inter-layer A (1) 1s exciton at the K(K ) point 12 and to the indirect band edge states. The relaxation channel can be further aided if the bilayer WS 2 is stacked on a few layer graphene, due to ultra-fast inter-layer carrier transfer processes [50][51][52][53][54] compared with device S2-F, requiring a higher electric field to obtain a similar Stark shift.…”